In imaging methods like electro(photo)graphy, magnetography, ionography, etc. a latent image is formed which is developed by attraction of so-called toner particles. Afterwards the developed latent image (toner image) is transferred to a final substrate and fused to this substrate. In direct electrostatic printing (DEP) printing is performed directly from a toner delivery means on a receiving substrate by means of an electronically addressable print head structure.
Toner particles are basically polymeric particles comprising a polymeric resin as a main component and various ingredients mixed with said toner resin. Apart from colorless toners, which are used e.g. for finishing function, the toner particles comprise at least one black and/or coloring substance, e.g., colored pigment.
In toner development of latent electrostatic images two techniques have been applied: “dry”, powder development and “liquid” dispersion development. Dry powder development is nowadays most frequently used.
In dry development, the application of dry toner powder to the substrate carrying the latent electrostatic image or the latent magnetic image may be carried out by different methods, including cascade, magnetic brush, powder cloud, impression, and transfer or touch down development methods. In liquid development, the toner particles are suspended in an insulating liquid, both constituents forming together the so-called liquid developer. During the development step, the toner particles are deposited image-wise on the latent electrostatic image-bearing carrier or the latent magnetic image-bearing carrier by electrophoresis (under the influence of electrical fields) or magnetophoresis (under the influence of magnetic fields). In these particular development steps, the toner particles have, respectively, an electrical charge or a magnetization.
Recent progress in digital printing methods makes considerations such as cost per copy, layer thickness of the marking material, resolution, and speed of imaging extremely important. In this respect, liquid toner systems have marked advantages over dry toner imaging techniques because the imaging particles are much smaller in size (compared to dry toner particles) and are comparable in size to typical conventional ink layer thicknesses. A liquid toner composition is for example disclosed in EP-A-1 341 053.
The visible image of electrostatically or magnetically attracted toner particles is not permanent and has to be fixed. Fixing is accomplished by causing the toner particles to adhere to the final substrate by softening or fusing them, followed by cooling. Typically, fixing is conducted on substantially porous paper by causing or forcing the softened or fused toner mass to penetrate into the surface irregularities of the paper.
There are different types of processes used for fusing a toner image to its final substrate. Some are based on fusing by heat, others are based on softening by solvent vapors, and others by the application of cold flow at high pressure under ambient temperature conditions. After the operation of being produced, the toner images further have to withstand some external forces applied during the subsequent treatments. The problems associated with multiple, superimposed layers of toner particles that are in one way or another fixed on a substrate are manifold, not only with respect to image quality but also with respect to image stability and with respect to mechanical issues.
An example of high mechanical impact on the toner layers is the sorting of printed papers. The fast turning wheels of a sorting machine can give a temperature increase above the glass transition temperature (Tg) of the resin used, that can cause contamination with pigmented toner resin on the next coming papers. Another application where the heat and mechanical resistance of the toner layer is stressed is the production of e.g. car manuals.
When the temperature inside the car rises above the Tg of the toner resin (e.g. when parked in the sun), the papers in the manual can stick to each other.
In the case of printing packaging materials with the use of toner technology, increased temperatures are met in many ways. Plastic can be used as a substrate and bags made out of it with the use of a sealing apparatus. If the sealing temperature is above the Tg of the toner resin used, the toner images get disturbed.
For a lot of these applications, a toner resin with a higher Tg should be used, but then the amount of energy necessary to fuse the toner particle onto the substrate would be so high that the application is energetically not interesting anymore. Furthermore, a lot of substrates can't be used anymore. High Tg toners exist already, but the demand for high speed engines increases the demand for toner particles which can be fused at normal fusing temperatures at a very high speed.
A lot of new applications are emerging. Especially in the pharmaceutical and in the food industry, there is an increased need for correct product information and for traceability. Product information related to expiration date, origin of the product, batch number is becoming more and more important. This induces the need for variable data printing down to the level of single items. Information is not only to be printed on the overall packaging, but also on the individual wrap. The printed data should be erase proof. Curable toner would offer an interesting concept.
The toxicity of a reactive and curable system however imposes limitations. In the case of dry curable toner technology, all components are contained within the toner particles, reducing the migration of toxic components. Liquid curable toners would however be more interesting as they enable to print at higher resolution and at lower cost. However the migration of the active components in liquid curable toners is an issue.
Adding curable additives or the use of a curable dispersant for the liquid toner is not suited, especially in applications involving food, pharmaceuticals, etc. Thermal curability is also not so suited, since it occurs at higher temperatures and involves long residence times, prohibiting printing on the product as such and or the use of temperature sensitive packaging material. In this sense UV curability is more suited than thermal curability, however with the restriction that the active components are to be not free to dissolve or migrate.
From the discussion it is obvious that there is a need for a low temperature, UV curable liquid toner, showing no migration of the active components, nor having presence of active components in the dispersant.
Radiation curable dry toners as known for example from EP-A-1 437 628 and WO 2005/116778. In these toners the resin is cured either in-line, e.g. at the time of fusing the toner to a substrate or off-line, e.g. after fusing the toner to a substrate. Curing of the resin can be conducted by radiation, such as UV-radiation, electron beam or chemically. By curing the resin the toner becomes permanently fixed to the substrate and the problems associated with non-curable toners in particular when the printed substrates are used under high temperature conditions are met.
In view of the above described advantages of liquid toner systems over dry toner systems there is, however, still a need for an improved curable liquid developer composition. Unfortunately, to provide a curable liquid developer composition turned out to be difficult because the required initiator may dissolve in the liquid dispersant and the polymeric dispersing agent which assists the dispersion of the toner particles in the dispersant may hinder the cross-linking of the toner particles. Thus, there is still a need to provide a radiation curable liquid toner which can be fixed at low temperatures but which is resistant to high temperatures once printed while maintaining all the other properties necessary to function correctly in a printer.